A variety of extensional and contractional structures is produced by strike slip faulting. The variety and extent of the structures are directly related to the kind and extent of geometric complexities of the fault zone or system. The area of convergence of the San Andreas fault zone and the much younger San Jacinto fault zone in the eastern Transverse Ranges is exquisitely complex. We propose that the San Jacinto fault zone formed in response to a structural knot in San Gorgonio Pass probably within the past 1.5 Ma. In the area of their convergence we propose that slip is transferred both east and west from the San Jacinto fault zone northward to the San Andreas fault zone over a 60-to 70-km band that extends northwestward from the south end of the San Bernardino basin to the east end of the San Gabriel Mountains. We further propose several structural adjustments as a consequence of onset or acceleration of lateral movement on the San Jacinto fault zone: accelerated uplift of the eastern San Gabriel Mountains, development or accentuation of an arcuate schuppen-like structure in the eastern San Gabriel Mountains, inception of the San Bernardino basin, cessation of deposition in the present-day San Timoteo badlands area, inception of the San Jacinto basin, and an increase in compression and uplift in the San Gorgonio Pass area. We interpret the uplift and compression in San Gorgonio Pass to result from two formerly disparate structural blocks—the eastern San Bernardino and San Jacinto blocks—becoming a relatively coherent block, and the San Gorgonio Pass area constituting a left step between the San Andreas fault zone in the Coachella Valley area and the San Jacinto fault zone in the San Jacinto Valley area. The compression and uplift led to the formation of the San Gorgonio Pass thrust faults and disruption of any through-going San Andreas strands, at least at the surface. In partitioning slip between the San Andreas and San Jacinto fault zones, consideration should be given to the bandwidth over which horizontal strain has accumulated. The average slip rate of the northern part of the San Jacinto fault zone during the past 1.5 m.y. may have been about 20 mm/yr and about 15 mm/yr on the San Andreas. South of the San Bernardino basin, current strain accumulation based on repeated geodetic surveys is nearly equally divided between the San Jacinto and San Andreas fault zones.

ABSTRACT The San Andreas Fault System — hereafter SAFS — has three major segments with notably different characteristics ( Figure 1 ). The Northwestern Segment extends from the Mendocino triple junction southeast for 720 km, with Salinian crystalline rocks on the west paradoxically abutting oceanic Franciscan sediments and volcanics on the east. The 275-km-long Central, or Big Bend, Segment has essentially identical crystalline rocks contiguous on either side. The 330-km-long onland part of the Southeastern Segment encompasses the Salton Trough, also has crystalline rocks on both sides, and has a high geothermal gradient typical of crustal spreading centers. This is the only segment of the SAFS that appears to be a crustal plate boundary. This study is based on a review and analysis of a formidable array of published data on the Big Bend Segment of the SAFS and of relevant geology to both its north and south (Appendix A, page 31). Its conclusion is that only modest strike-slip displacement, on the order of 15 to 50 km, has occurred in the Big Bend Segment. Areal geology displays abundant evidence of lateral shortening, including thrust and reverse faulting and mountain building whose vertical magnitude is comparable to horizontal displacements. The nature of the San Gabriel fault is also analyzed here because of past wide acceptance of it as an ancestral SAFS during Early Pliocene time, which produced a claimed right-lateral offset of about 48 km. The conclusion here is that both the proposal of a one-time connection of the San Gabriel fault with the SAFS and a 48-km displacement on the former are unwarranted. The conclusions of this paper stand in contrast to the multiple conflicting claims of major right-lateral offsets. Also, geologists have commonly expressed opinions of the timing and extent of displacement without acknowledging that they differed notably from earlier estimates.